The output signals of many sensing devices, e.g. piezoelectric accelerometer or photodetector, are current signals. The piezoelectric accelerometer is often used for the measurement of pressure signals. The photodetector is often used to convert incident light to a current. The photodetector has been widely applied in medical instrumentation, laboratory instrumentation, position and proximity sensors, photographic analyzers, barcode scanners and smoke detectors. Because the electrical techniques of processing current signals are not so popular as those of processing voltage signals, the post processing circuit of the current-type sensing device is usually a current-to-voltage converter. However, the measured signal is often corrupted with dc or low-frequency interference with large magnitude. Hence, in order to obtain a desired better suppression of unwanted interference in the applications of current-type sensing devices, a new circuit is presented in this invention for the realization of a current-to-voltage converter with highpass filter function.
In the literature, the monolithic combination of photodiode and transresistance amplifier on a single chip has been produced for the measurement and instrumentation applications by the Burr-Brown (BB) Corp. A cascode design for the photodetector with a highpass filter has been developed by the BB Corp. to reject ambient light, yet provide high ac gain for best signal-to-noise ratio. However, the photodetector with highpass filter function recommended by the BB Corp. needs six passive elements and requires the symmetric condition of passive elements. In this invention, we also propose a new circuit for photodetector to reject ambient light with the utilization of only three passive elements. The proposed circuit enjoys the advantages of the independence of -3 dB frequency and the gain, and no constraint of requiring the matched elements.